1. Technical Field
The present disclosure refers to satellite positioning systems and particularly techniques for acquiring CDMA-type satellite signals.
2. Description of the Related Art
The satellite signals used in the GNSS (Global Navigation Satellite System) are of the CDMA-type (Code Division Multiple Access). The satellite signals reception at the receiving apparatus provides the following sequentially performed standard steps: frequency conversion and digitization, acquisition, tracking, decoding and positioning.
A radio-frequency stage processes the analog signals received at the satellites and converts them at an intermediate frequency, and an analog-digital converter converts the intermediate frequency signals to corresponding digital signals.
The intermediate frequency-converted signal has frequency offsets due to offsets of the local oscillator used for the conversion and due to Doppler effects caused by the motions of the satellites and receiving apparatus.
In the acquisition of CDMA signals from satellites, there are two main operative states: the “Cold Start” state and the “Hot/Warm start” state.
In the “Cold Start” state, it is assumed that the Doppler shift due to the satellite and receiving apparatus' motions is completely unknown.
In the “Hot/Warm start” state, a high-point (e.g., maximum) Doppler uncertainty at the reception is considered null because in such state it is possible to predict both the satellites in view and their velocities by assuming a known position (corresponding to the position available at the moment of the previous switching-off) and the time (generated by the inner backup clock).
With reference to the “Hot/Warm start” assumption, the acquisition step, managed by a corresponding acquisition block, performed for the generic satellite, provides a calculation of a plurality of correlations among the intermediate frequency-converted signal from the satellites, and locally generated test signals.
Such test signals are generated by performing a frequency scan in a frequency range which should take into account the frequency offsets of the local oscillator and Doppler shifts. Moreover, the test signals are also generated by a phase or code position scan providing the phase shift of a locally generated pseudo-random replication code.
The calculation of each correlation is performed by a numerical integration on a band exclusively related to an elementary integration period adopted by limits set by the Nyquist theorem.
The correlator output associated to any possible phases or code positions and on each search frequency scan (bin) can be generally considered as a complex signal, in other words an associated power information.
Document U.S. Pat. No. 7,403,558 describes an acquisition technique enabling to speed up the correlations calculation.
After the correlations calculation, the individuation of the code-\l frequency pair taking to a maximum correlation value is performed.
Techniques intended to improve the performances in the Hot/Warm start state are known. For example, it is considered the paper by David M. Lin et al. “Sensitivity Limit of a Stand Alone GPS Receiver and An Acquisition Method”, ION GPS 2002, 24-27 Sep. 200, Portland, Oreg., which discusses some modes intended to increase the sensitivity of the receiving apparatus, and analyzes two approaches.
The reception sensitivity can be quantified by the “minimum signal/noise ratio Q” parameter defined as the low-point (e.g., minimum) signal/noise ratio associated to the signal from the satellite as received at the receiving apparatus and detected by a probability of detection (POD), wherein POD is a project value.
Referring back to the paper by David M. Lin et al., according to the first approach, in order to increase the sensitivity of the integration scan, it is performed a coherent integration on a longer time range, in other words it is increased the duration of each integration step.
The second approach is based on a combination of coherent and incoherent integrations. The second approach provides that more integration steps are done, that is steps of correlation calculation, at the same frequency as the test signal and in cumulative time ranges. The results of these integrations are accumulated before evaluating which is the selected candidate.
It is demonstrable that each time the number of consecutive and disjoined (or incoherent) integration steps executed at the same frequency is doubled, the scan sensitivity is increased by 1.5 dB.
Moreover, the above mentioned paper by David M. Lin et al. demonstrates that the scan sensitivity, given a fixed number of total steps, improves of 3 dB each time the duration of an elementary step is doubled.
It has been observed that the coherent integration technique reduces the elementary band covered by the work scan (band which, according to the Nyquist theorem, is therefore halved each time the coherent time is doubled) and therefore it is subjected, when the correctness assumption of frequency fails, and given an initial determined uncertainty, to the scan of a greater number of potential bins and to a total typical greater time.
Moreover, it has been observed that the coherent integration cannot be directly used in the Hot/Warm start state because when the searched frequency is outside the work bin, due to a prediction error, the dynamics of the receiving apparatus, or the drift of the local oscillator, would make the receiving apparatus vulnerable to alias phenomena. In other words, the satellite signals transmitted would not be hooked and tracked but instead, the replications generated by the side lobes of the spectrum itself would be the ones which fall in the search band. Such side lobes, although weaker than the main lobe signal (typically 30 dB for a GNSS), can be practically acquired by high sensitivity scans performed on erroneous bands and at the end will be erroneously detected as a main lobe to be tracked with consequent frequency and position errors.